The present invention concerns circuits used for communication systems and pertains specifically to differential filters with common mode rejection and broadband rejection.
For applications such as cellular phones, it is desirable to reduce the size of components. Particularly, it is desirable to integrate RF duplexers and filters as part of a radio-on-a-chip with a readily manufactured technology.
Bulk acoustic resonators have been used to implement filters. One advantage of using acoustic resonators is that the speed of sound is approximately three or four orders of magnitude smaller than the speed of light, making the wavelengths, and thus the dimensions of a device, small compared with conventional (L-C) tank circuits.
There are two broad classes of circuits: single ended and balanced. Single ended filter circuits respond to a signal that is referenced to ground. Balanced filter circuits respond preferentially to signals that reference to each other.
There are numerous ways to balance circuits. For example, a circuit structure can be built with three phases (modes). Three phase power delivery is done in one of two manners, Wye (Y) and delta (xcex94). For delta manner phase power delivery, power is delivered on three lines. There is no ground node. All voltages are defined with respect to each other. Each of the signals on the three lines is 120 degrees out of phase with respect to the signals on the other two lines.
For Wye manner phase power delivery, a ground is located close enough to induce currents from the transmission lines. When a single phase is pulled off, a parasitic Wye mode becomes defined. So, Wye mode can be used to deliver single ended power out of a power line. However, a Wye mode can be formed as a parasitic mode in lines formed in a Wye formation. It is also possible to build a balanced circuit structure with only two phases. Such a circuit is called a differential circuit. In this case, signals on two lines are 180 degrees out of phase. When the two signals are not balanced, then the extra energy shows up as a single ended term called the common mode. Effectively, each of the otherwise out-of-phase signals have some inphase energy as well. The common mode does not have to be the same frequency as the differential mode.
There are two sources of common mode. The network itself may not be symmetric, with the resulting unbalance causing common mode. This is typical in a passive structure. And if the input signal is not completely balanced then there will be common mode in proportion to this lack of balance. This lack of balance at the input can be compensated for within the network, as is typical with an integrated circuit, such as a differential amplifier. Specifically, this balance can be restored through common mode rejection. For a general discussion of common mode rejection, see for example, Paul Gray and Robert Meyer, xe2x80x9cAnalysis and Design of Analog Integrated Circuitsxe2x80x9d, 2nd edition, Wiley, 1977, 1984.
There are equivalently single ended and balanced bandpass filters. There are many forms of single ended filters. If limited to two dimensional, resonator based, such as filters that use film bulk acoustic resonators (FBARs), the number of forms is greatly reduced. The basic structure is a half ladder.
A filter using a half ladder structure can be significantly enhanced through various forms of coupling. Coupling paths can be around series elements. Coupling paths can be between shunt elements. Coupling paths can be between series and shunt elements. The coupling can be either capacitive or inductive. The capacitive coupling is due either to the proximity of printed metal lines, by design or as an unwanted parasitic, or from a directly formed capacitor. The inductive coupling can be the result of both bond wire and printed metal line proximity, also either by design or as an unwanted parasitic. These modifications can modify the slope and passband width, but do not change the basic shape of the filter response.
A similarly fabricated differential filter typically has one of three main structures. The first structure is a pair two identical half ladders (also called a paired half ladder structure). A second structure is a full ladder structure. A third structure for a differential filter is a lattice structure.
Each different differential filter structure has advantages and disadvantages. The frequency response of a differential filter with a ladder structure is quite different than the frequency response of a differential filter with a lattice structure. A differential filter with a ladder structure tends to have a very steep rejection response, followed by a return to less rejection. A differential filter with a ladder structure is typically quite effective in blocking signals close to the passband, but poor at rejecting further frequencies. A differential filter with a lattice structure rejects very well for frequencies further from the passband, but not well for frequencies closer to the passband.
Differential filters have both differential and common mode responses. Differential filters with full ladder structures and differential filters with lattice structures generally do not respond well to common mode signals. Differential filters with full ladder structures and differential filters with lattice structures are completely symmetric, and so do not contribute directly to common mode. However, differential filters with full ladder structures and differential filters with lattice structures have no provision for rejecting common mode that is already contained within the input signal. Typically, there is significant imbalance at the input to any network.
Differential filters with paired half ladder structures reject common mode directly. However, differential filters with paired half ladder structures are really two separate filters and it is difficult to balance two separate filters. The imbalance results in more common mode. The difficulty in balancing two half ladder structures is compounded by the nature of a half ladder structure. Each shunt element is grounded, individually. The ground paths are dependent upon geometry, and so it is difficult to make these ground paths identical. The inductance in the path to ground has an effect, reducing the slope of the transition from passband to reject band. This effect can be either useful or detrimental, depending upon the filter requirements.
In accordance with the preferred embodiment of the present invention, a differential filter includes a first input, a second input, a first output, a second output, and a plurality of acoustic resonator elements. The plurality of acoustic resonator elements is connected to the first input, the second input, the first output and the second output. The acoustic resonator elements are arranged to form both a lattice structure and a full ladder structure. In alternate embodiments of the present invention, the acoustic resonator elements are arranged to form both a full ladder structure and a paired half ladder structure, to form both a lattice structure and a paired half ladder structure, or to form all of a lattice structure, a full ladder structure and a paired half ladder structure.